It is known that printed wiring boards can be developed on a flexible (film) or rigid (laminate) substrate covered by a conductive metal such as copper foil. The majority of printed wiring boards, sometimes hereinafter referred to as "PWB's", have two layers covered with circuitry. Components such as integrated circuits, capacitors and resistors are mounted on one side of the board. The board has at least one power and ground terminal. The components are interconnected by copper conduits. In order to save space, on a two-sided board the conduits run parallel with no intersection of conduits being allowed. When a cross-connection is necessary, three avenues of approach are possible: (1) expand the circuitry and board surface to accommodate the winding lines; (2) install an insulating bar with a conduit on the top that runs across the conduits (bus-bar); or (3) make a "multilayer" construction. Multilayer constructions provide high density circuitry as well as cross-connections between layers.
Conventional multilayer PWB's are manufactured by a relamination process wherein a laminate circuitry is developed and then either several of such laminates are bonded together (pin relamination) or additional layers of dielectric and copper are laminated on top of the "core" and subsequently multiple circuitry is developed. The interconnecting of the planes is achieved by drilling holes through the finished multilayer laminate and by plating the holes to provide conduits. The circuit design of the individual planes is such that, when aligned, the drilled hole will pass through a solid copper pad for the instances in which a connection has to be made or, through an etched space (hole) for the instance where no connection is desired. Typically individual circuit planes are designed to be ground or power planes only. If a multiple power supply is required, additional planes are necessary. The steps necessary in the process make it lengthy and expensive.
According to the present invention, there has been developed a method of manufacturing three-layered laminates for printed wiring boards in which the design of the center copper plane makes it unrestricted and universal in regard to the size of the PWB that will be cut out of the sheet and to the intended possible multiple use of the center layer. The manufacturing process is a one-shot sandwich lamination in which a prepunched or photochemically developed self-sustaining center copper sheet is pressed between layers of glass fabric which is impregnated with resin (prepreg). On the outside of the laminated stack are two layers of solid copper foil, preferably that have been suitably treated to promote adhesion to the prepreg. The design of the center copper layer has a recurring series of patterns of holes and is such that any size PWB or combination thereof can be cut from the laminate. The center copper layer can be used as power, ground or combination plane, or as a heat sink. The alignment of the center layer grid-work pattern and the eventual design of outer circuitry and ensuing hole drilling is accomplished through the use of an overlay sheet. The circuit designer is thus afforded great flexibility in the location of ICs, resistors and other elements.